1. Field of the Invention
The present invention relates to an inkjet recording system realizing improved ink charging ratio (i.e., ratio between discharge nozzle number/total nozzle number) in discharging pigment ink through a liquid droplet discharge head, and providing excellent image quality.
2. Description of Related Art
Conventionally, inkjet recording apparatuses serving as image forming apparatuses such as printer, facsimile machine, copying machine and plotter have been known. A liquid droplet discharge head used in such an inkjet recording apparatus includes a nozzle for discharging a liquid droplet, a liquid chamber (also referred to as discharge chamber, pressure chamber, pressurizing liquid chamber, ink flow channel) communicating with the nozzle, and an actuator that generates energy for pressurizing recording liquid (ink) in the liquid chamber, wherein a liquid droplet is discharged from the nozzle by causing pressure to act on the recording liquid in the liquid chamber by generating energy.
As the liquid droplet discharge head, some apparatuses use a piezoelectric actuator such as electromechanical transduction device, some apparatuses use a thermal actuator utilizing film boiling for electro-thermal transduction device, and other apparatuses use an electrostatic actuator utilizing a vibrating plate and electrostatic force between electrodes.
As the inkjet recording ink, aqueous dye ink in which water-soluble dye is dissolved in aqueous medium has been used because it has high staining power and causes little clogging in a head nozzle.
However, such aqueous dye ink has the problem of insufficient water resistance and weather resistance. From this point, pigment ink having better water resistance, weather resistance compared to dye ink has been increasingly used.
On the other hand, for high-speed printing, a line head system is more suited than a conventional serial head system, however, it is necessary to discharge an ink droplet at a velocity larger than a lower limit value at which sheet conveying velocity has no influence. As a measure that causes ink to be discharged at the velocity larger than that lower limit value, increasing the driving voltage of the actuator can be exemplified. However, with the increase of driving voltage, the load applied onto the actuator increases. This is not desired because the life time of the printer head is shortened, and also from the viewpoint of energy saving.
As a technique concerning maintenance of discharge velocity and recovery of discharge in the line head system, a recording apparatus that has means of controlling ink discharge velocity by controlling either of viscosity of ink, specific gravity, or concentration of solids in the ink is disclosed in Japanese Unexamined Patent Application Publication No. 10-217478. Also a recording apparatus having means of recovering disorder of liquid discharge from a nozzle for each individual head unit is proposed in Japanese Unexamined Patent Application Publication No. 8-127137.
However, when this liquid droplet discharge head is filled with pigment ink using a multi-nozzle liquid droplet discharge head in which a part of wall surface of pressure chamber is formed of a piezoelectric element, non-discharge nozzles occur more frequently compared to the case where dye ink is used, so that troubles such as jet disability, dot missing and disorder of printing occur, leading impairment in printing quality.
As described above, for responding to high-speed printing, it is necessary to discharge ink at a velocity larger than a certain lower limit velocity at which no influence is exerted by sheet conveying velocity. The higher the printing speed, the larger the lower limit velocity should be set. Therefore, in order to make a liquid droplet land at a predetermined position on the recording sheet, the velocity at which the ink droplet lands is preferably 8 m/s or larger, and more preferably 9 m/s or larger. Further, in order to form an image of high quality, it is desired that no non-discharge nozzle occurs, and variation in discharge velocity between nozzles is small.
However, it was demonstrated that when pigment ink was discharged using the above-described multi-nozzle liquid droplet discharge head, the discharge velocity of pigment ink was smaller than that of dye ink, and hence the desired 9 m/s was not realized.
In other words, when an image is formed by using pigment ink, it is impossible to discharge ink at a discharge velocity larger than the lower limit value at which sheet conveying velocity does not influence (9 m/s or larger). Therefore, lines or uneven coloring may significantly occur in association with deviation at the time of landing of ink droplet, so that image quality is impaired.